High Electron Mobility Transistors (HEMTs) were first developed in the 1970's and are employed in advanced communications applications, such as microwave circuit applications. The first HEMTs were formed using GaAs and AlGaAs, but new structures using GaN and other materials are becoming of greater importance. The HEMT behaves much like a conventional Field Effect Transistor (FET), and the fabrication of HEMT devices is based on FET architecture. However, HEMTs require a very precise, lattice-matched heterojunction between two compound semiconductor layers.
GaN Field-effect devices are piezoelectric devices, i.e., the presence of spontaneous and piezoelectric polarizations are responsible for charge separation within the material. This has the enormous advantage in that it allows, at the interfaces between different material compositions, e.g. AlGaN and GaN, the creation of surface charged layers in the structures. Those surface charged layers are compensated in the AlGaN/GaN junction by the formation of two-dimensional electron gas (2DEG) at the other side of the interface, which has high mobility values compared to the bulk material. While necessary and beneficial for the creation of a very high mobility channel in the structure, allowing for operation of high power HEMT devices, it has the drawback in that the separation charges within a wurtzite material are also responsible for the surface charged layer at the top surface of the structure, leading to an increased importance of surface states, compared to zincblend structure materials. The presence of surface states creates acute problems for achieving the high performance that is theoretically predicted for those devices, as surface states play an important role during device operation. It has been noted by many authors that the reduction of DC performance, current slump at high drain-source voltages, and DC to RF dispersion phenomena are directly related to the filling and emptying of surface states, although thermal effects are also partially responsible for DC current slump.
To minimize the effect of such surface states on the top III-N structures, passivation of the surface between drain and source by, e.g., SiN or SiO2, has been proposed by different authors, using essentially ex-situ methods during the processing of the transistors. These methods are very dependent on the growth conditions of the oxide or insulating layer on the top surface, as strain effects, tuned by growth conditions and also dependent on the strain state in the heterostructure itself, also have a influence on the two-dimensional electron gas properties. They are also dependent on the chemical or mechanical state of the surface, which is dependent on the previous processing steps that have occurred on this top surface.
US 2003/0020092 Al describes an AlGaN/GaN HEMT having a thin AlGaN layer. Source and drain contacts contact the AlGaN layer, while parts of the AlGaN layer are uncovered by the contacts. An insulating layer covers the uncovered part of the AlGaN layer and a gate contact is included on the insulating layer. In an embodiment, the HEMT and the insulating layer are fabricated using metal-organic chemical vapor deposition. In another embodiment, the insulating layer is sputtered onto the top surface of the HEMT active layers.
WO 01/13436 Al describes a GaN based FET that employs dielectric passivation layers on exposed AlGaN or GaN surfaces of the devices above the channel regions, between the source and drain contacts. The dielectric layer is formed of SiN. The layer controls undesirable frequency-dependent current and reduced breakdown voltage.
U.S. Pat. No. 5,192,987 discloses a high electron mobility transistor. The transistor consists of a GaN/AlGaN heterojunction where a two-dimensional electron gas occurs. The structures are deposited on basal plane sapphire using low-pressure metal organic chemical vapor deposition.
Another problem in the development of field-effect GaN based transistors is the ohmic contact formation on the group III-nitride surface. The ohmic contact formation is dependent on different factors, e.g., the surface composition, but one very important factor, which has long been underestimated, is the degree of oxidation of the top surface. Different cleaning techniques and metallization techniques have been suggested to overcome this problem. The lack of a good ohmic contact formation on the top layer directly leads to drastic reduction of the device performance. It is very often observed that the maximum current which is measured, under DC conditions, is well below the maximum current density values deduced from material considerations such as carrier density and mobility in the channel.
Furthermore, the uniformity and reproducibility of the device performances over a wafer and from one wafer to another is often a problem. Even if record performance has been demonstrated on selected HEMT devices, uniformity and reproducibility of the results remain problematic. Although improvement of material quality and device processing is believed to allow for better results in uniformity and reproducibility, a fundamental problem to be overcome is the accurate control of the surface properties.
Finally, another problem frequently encountered with piezoelectric devices such as AlGaN/GaN HEMT devices is encountered. In order to increase the current density in the two-dimensional electron gas, there exist two possibilities: either increase the AlGaN layer thickness, or increase the Al content in this top surface. These two possibilities, by increasing the strain in the top AlGaN, which is grown pseudomorphically on the GaN layer, an increased carrier density in the channel can be obtained. However, the presence of high strain in the AlGaN layer rapidly leads to cracking of this top surface. Those cracks in the AlGaN are very prejudicial—they destroy the 2DEG at the AlGaN/GaN interface; second, they complicate processing.